For performing an optical measurement of depth, it is frequently desirable to derive a range image, which represents the distances between a viewing point and various surfaces in a scene. Range images may be derived by active techniques in which some form of energy is directed toward an object and then measured on its return. Popular methods of active ranging include sonar, laser rangefinding, and structured light. Range images may also be derived by passive techniques, in which the light from a normally illuminated scene is captured and analyzed by one or more television cameras. Of the passive techniques, binocular stereo is the most popular.
There are many situations in which active techniques are inconvenient or ineffective in acquiring the desired range imagery. The most popular passive technique, binocular stereo, has a number of disadvantages as well. It requires the use of two cameras that are accurately positioned and calibrated. Analyzing the data involves solving the correspondence problem, which is the problem of determining the matches between corresponding image points in the two views obtained from the two cameras. The correspondence problem is known to be difficult and demanding from a computational standpoint, and existing techniques for solving it often lead to ambiguities of interpretation. The problems can be ameliorated to some extent by the addition of a third camera (i.e. trinocular stereopsis), but many difficulties remain.
The correspondence problem can also be avoided if one acquires a series of images from a series of closely spaced viewpoints, as is done in epipolar analysis. However, the procedure in this case is quite cumbersome, since a camera must move along a trajectory over an extended period of time in order to gather a sequence of images.